Hydra is a well-known small animal of the order Cnidaria. They are diploblastic animals. They are free-living animals and are known as freshwater polyps. Swiss scientist Abraham Trembley (1700-1784) discovered Hydra. Carolus Linnaeus (1758) named it Hydra after the nine-headed dragon Hydra in Greek mythology. Cutting off one head of the dragon hydra would produce two heads. If any part of it was torn off, a new hydra would be created from that part. That is, hydra has the ability to regenerate. Mahavir Hercules killed the monster Hydra. Dr. Siddiq Publications
Hydra’s identifying features
(i) Their bodies are long, cylindrical and slender.
(ii) The upper part of the body is open and the lower part is closed.
(iii) Twisted hypostome is located at the free end of the body.
(iv) Hypostome has 6–8 scutes.
(v) One or more buds are present on the body
(vi) At the lower end of the body is the foot-disc.
Habitat of Hydra
Different species of hydra are found all over the world. Lives in freshwater bodies such as ponds, ponds, canals, bilges, lakes, springs etc. They are not found in turbid, high and moving water. They hang downwards on a submerged solid object or plant leaf. Hydra vulgaris (orange-red), Hydra oligactis and Hydra viridissima (green) are found in freshwater bodies of Asia, Europe and America, including Bangladesh.
Food of Hydra
Hydra is carnivorous animal. Hydra feeds on insect larvae, small insects, cyclops, daphnia/crustacea, small worms, annelids, fish eggs, frogs etc. But the main diet is crustacean joints. Absorption and excretion are accomplished by contraction and expansion of the body wall. Hydra only preys on animals that have glutathione in their collars.
Hydra species of Bangladesh
There are about 40 Hydra species found in the world. 3 species of hydra are found in Bangladesh. Colorless or yellow-brown Hydra vulgaris, brown Hydra oligactis (Hydra fusca) and green Hydra viridissima. Hydra vulgaris is found in abundance in freshwater reservoirs in Bangladesh. Dr. Siddiq Publications
External structure of Hydra
Hydra is a small tube-shaped animal. When expanded they are about 1-3 cm long and 1 mm wide. When removed from the water, they turn into a soft and shapeless lump. Hydra’s body consists of three parts. Hypostome, body stem and foot.
1. Hypostome: The word Hypostome is formed from hypo meaning below and stoma meaning mouth. The upper part of the body has a mouth opening. The small and twisted contractile organ below the stoma is called the hypostome. Stomata receive food, water and oxygen. It expels undigested or waste material.
2. Body: The entire body except the hypostome and the base is called the body. Different parts of the body-
(i) Sterile: At the base of the hypostome there are 6-10 slender, long, hollow and contractile-expanding sturgeon. Each karshika is twice or thrice as long as the body. The outer wall of Karshika consists of batteries of small nematocysts like tumors. Karshikas are arranged in a circle. It participates in food intake, movement and self-defense.
(ii) Mukul: Favorable environment or environment with sufficient food during summer. Hydra gains physical growth by consuming food. As a result, one or more buds are formed in the middle region of the body. Each bud matures and gives birth to a new hydra. This is one of hydra’s asexual reproduction methods. Dr. Siddiq Publications
(iii) Genitalia: During hibernation and winter, temporary genitalia are formed on the body of the metamorphosed hydra. The upper part of the body contains the spermatozoa and the lower part the ovary. The testicles produce sperm and the ovaries produce eggs. Participates in sperm and egg production.
(iv) Pedicel: The part between the stem and the foot is the petiole. It is narrow, narrow and contractile. It connects the body and the foot.
3. Padachakti: The round and pressed part in the lower part of the body is the sole of the foot. A sticky substance secreted from the soles of the feet keeps the hydra attached to objects. Bubbles help float in water. It helps the hydra to move by forming a membrane. Hydra displays gliding movements with the help of discs. Dr. Siddiq Publications
Internal structure of Hydra
Longitudinal or transverse section of Hydra showing the body wall and gastrovascular or coelenteron. The body wall consists of two cell layers. Epidermis (embryonic ectoderm) and gastrodermis (embryonic endoderm).
The outermost cell layer of the body wall is called the epidermis. The thickness of the epidermis is one-third that of the body wall. It contains seven types of cells. Muscle sheath cells, interstitial cells, sensory cells, nerve cells, glandular cells, germ cells and nidoblasts.
1. Musculo-Epithelial cells: Musculo-epithelial cells are located throughout the epidermis. The cells look columnar. The outside of each cell is wide and free and the inside is closed and narrow. There are two muscle enhancers on the inside. It is also called muscle tail. Within the muscle tail are the contractile fibers myoneme. The cytoplasm of the cell contains a nucleus, mucous material and granular glands. The cells lining the musculature are large, flattened and contain nidocytes. Mucin is secreted from the mucous material and granules from the granular glands. Muscle sheath cells are interconnected to form an integral sheath.
Musculoskeletal function
(i) Musculature protects the body by creating a body covering.
(ii) Myoneme causes contraction-expansion of the body.
(iii) Mucous gland secretion keeps the body slippery.
(iv) It bears nematocysts.
(v) It helps in movement by contraction-expansion.
(vi) Mucous granules secreting sap form the cuticle.
(vii) It contains nidoblasts.
(viii) Keeps the body attached to an object.
(ix) Part rich in mucous granules participates in digestion.
2. Interstitial cell: Interstitial cells are located in clusters between muscle cells. It is called Reserve cell or Stem cell or multipotent cell. The cells appear round, oval or triangular in shape. Each cell contains a nucleus, numerous mitochondria, endoplasmic reticulum, lysosomes, ribosomes etc. It has totipotency capability. So it transforms into any other cell as needed. The diameter of the cells is 5 μm.
Function of interstitial cells
(i) A cell can transform into any other cell.
(ii) It participates in hydra regeneration, growth, gonad and bud formation.
(iiii) After 45 days, when body cells are destroyed, interstitial cells fill the space.
3. Sensory cell: Sensory cells are scattered in the spaces between the muscle covering cells. However, it is numerous in karshika, hypostome, feet. The cells appear narrow, elongated and spindle-shaped. That is, the middle part is wide and both ends are narrow. Each cell has cytoplasm and a swollen nucleus. It has contractile membranes on the outside and sensory nerves on the inside. It is sensitive to light, heat, touch, chemicals etc. Dr. Siddiq Publications
Function of sensory cells
(i) It receives different types of stimuli from the environment.
(ii) It participates in self-defense.
(iii) It helps in selection of accommodation.
(iv) It selects food.
4. Nerve cells: Nerve cells are located under the epidermis near the mesoglia. The cells appear maculate or polygonal. Cytoplasm and nucleus are present in the cell. Each cell has two or more branched nervous systems. Nervous systems combine to form the neural network. Dr. Siddiq Publications
Nerve cell function
(i) It receives nerve impulses from sensory cells and generates reports.
(ii) It facilitates coordination between different cells.
5. Gland cell: Gland cells are located in the peduncle, hypostome and peduncle. Cells may appear cylindrical, granular, or oval. It contains mucous glands, enzyme glands and glands that secrete mucus.
Function of glandular cells
(i) A cell helps in digestion of food.
(ii) The secreted sticky juice keeps the hydra attached to an object.
(iii) It helps in locomotion by creating momentum.
(iv) Helps hydra to float in water by creating bubbles.
6. Germ cell: During the reproductive season, spermatozoa and ovum are formed from interstitial cells in the hydra body stem. The testes are above and the ovaries are below. The testicles produce sperm and the ovaries produce eggs. Sperm are very small and nucleated. It consists of head, midsection and movable tail. Eggs are large, round and have three polar bodies.
Function of reproductive cells: Sperm and egg participate in reproductive function.
7. Cnidocyte cell: Nidoblast cells are widespread everywhere except the soles of hydra. It is located in the spaces between the muscle cells. The cells look round, oval, flax-shaped, cupped or pear-shaped. Each cell is bilayered and contains only one nucleus. There are nematocysts with twisted threads inside the cells.
Nidoblast function
(i) It helps in food intake.
(ii) It helps in locomotion.
(iii) Participates in self-defence.
(iv) Captures and traps prey.
(v) Helps the animal to cling to an object.
(vi) It carries taxonomic importance.
Gastrodermis of Hydra
The innermost cell layer of the body wall is called the gastrodermis. The thickness of the gastrodermis is two-thirds that of the body wall. It contains 5 types of cells. Nutrient cells, glandular cells, interstitial cells, nerve cells and sensory cells.
1. Nutritive cells: Nutritive cells are located throughout most of the gastrodermis. It looks stunning. Each cell has a large nucleus and cavity. It forms contractile fibrous fascicles. Nutrient cells can be divided into two types. Flagellar cells and transient cells. Dr. Siddiq Publications
(i) Flagellar cells: These cells have 1-4 thread-like flagella at the free end.
(ii) Transient cells: The free end of this cell has transient cells.
Nutrients are cell functions
(i) It thins and thickens the body by contraction-expansion.
(ii) Flagella transform the food material into particles.
(iii) Acts like a sphincter to open and close the mouth.
(iv) It allows water to enter the mouth.
(v) Digests food.
2. Interstitial cell: Interstitial cells are located in clusters between muscle cells. It is called Reserve cell or Stem cell or multipotent cell. The cells appear round, oval or triangular in shape. Each cell contains a nucleus, numerous mitochondria, endoplasmic reticulum, lysosomes, ribosomes etc. It has totipotency capability. So it transforms into any other cell as needed. The diameter of the cells is 5 μm.
Function of interstitial cells
(i) A cell can transform into any other cell.
(ii) It participates in hydra regeneration, growth, gonad and bud formation.
(iiii) After 45 days, when body cells are destroyed, interstitial cells fill the space.
3. Sensory cell: Sensory cells are scattered in the spaces between the muscle covering cells. However, it is numerous in karshika, hypostome, feet. The cells appear narrow, elongated and spindle-shaped. That is, the middle part is wide and both ends are narrow. Each cell has cytoplasm and a swollen nucleus. It has contractile membranes on the outside and sensory nerves on the inside. It is sensitive to light, heat, touch, chemicals etc. Dr. Siddiq Publications
Function of sensory cells
(i) It receives different types of stimuli from the environment.
(ii) It participates in self-defense.
(iii) It helps in selection of accommodation.
(iv) It selects food.
4. Nerve cells: Nerve cells are located under the epidermis near the mesoglia. The cells appear maculate or polygonal. Cells have cytoplasm and nucleus. Each cell has two or more branched nervous systems. Nervous systems combine to form the neural network. Dr. Siddiq Publications
Nerve cell function
(i) It receives nerve impulses from sensory cells and generates reports.
(ii) It facilitates coordination between different cells.
5. Gland cell: Gland cells are located in the peduncle, hypostome and peduncle. Cells may appear cylindrical, granular, or oval. It contains mucous glands, enzyme glands and glands that secrete mucus.
Function of glandular cells
(i) A cell helps in digestion of food.
(ii) The secreted sticky juice keeps the hydra attached to an object.
(iii) It helps in locomotion by creating momentum.
(iv) Helps hydra to float in water by creating bubbles.
Mesogloea
The jelly-like sticky extracellular layer between the epidermis and gastrodermis of Nidaria is called mesoglia. It is thin, colorless and elastic. Its diameter is 0.1 micron. Dr. Siddiq Publications
Function of mesoglia
1. Mesoglia serve as the foundation of the epidermis and endoderm.
2. It acts as an attachment point.
3. It makes the body contract and expand.
4. Acts as a flexible skeleton of the body.
5. It contains myofibrils.
Cnidoblast or Cnidocyte
The word Cnidoblast is derived from the Greek words knide meaning nettle and blastos meaning germ. The goblet or flax-shaped cells on the exterior of hydra are called nidoblast cells. It can be round, oval, pear-shaped, cup-shaped or oval-shaped. Nidoblast cells are found everywhere in the body except the soles of the feet. But its number is more in Karshika. Sometimes the cells are arranged in clusters. A group of them is called a battery.
1. Integument: Each nidoblast cell is covered by a bilayered envelope. Outer cover and inner cover. It is composed of proteins and lipids. Between the two envelopes are granular cytoplasm, a nucleus, mitochondria, lysosomes, ribosomes, etc.
2. Nematocyst: Small cysts with twisted secretions present in nidoblast cells are called nematocysts. The sac of nematocyst is called capsule. Capsules contain the poisonous liquid hypnotoxin. Hypnotoxin is composed of proteins and phenols. Hypnotoxin poison is neurotoxic in nature. That is, it acts on the victim’s nervous system. The level of this venom is 75% similar to that of a gokra or cobra snake. The nematocyst has a long and hollow filament at its tip. The broad part of the base of the sutraka is called the butt or shaft. The butt has three large spines called barbs. The butt consists of spirally arranged small barbule spines. Normally, the nematocyst is inserted into the sac with butts and spines. A nematocyst once shed never reenters.Dr. Siddiq Publications
3. Operculum: The mouth of nematocyst has a lid like part. It is called operculum. It moves sideways when exposed.
4. Nidocile: The free end of the nidocyte cell consists of a tough, firm, tiny and hypersensitive hollow spine. It is called nidosil. It is a transformed cilium. It works like a trigger. As a result, patched sutraka is thrown out. Dr. Siddiq Publications
5. Muscle fibers and lasso: Several muscle fibers emerge from the lower end of the nematocyst. Besides, there is a twisted thread called lasso at the lower end.
Nidoblast function
1. Defense: Nematocysts contain a poisonous substance called hypnotoxin. Hydra defends itself with this toxic substance.
2. Prey capture: Hydra captures food by means of nematocysts. Then paralyze the victim. Dr. Siddiq Publications
3. Movement: Nematocyst cells help the hydra to move. Different types of movements occur in hydra. Dr. Siddiq Publications
4. Clogging the body: Nematocysts secrete sticky substances. By this, the hydra is stuck to an object. Dr. Siddiq Publications
Different types of Nematocysts in Hydra
Nematocysts are small sacs with patchy filaments located in nidocyte cells. Normally the sutraka is inserted into a sac or capsule with a butt and a fork. A nematocyst once shed never reenters. In 1965, scientist Werner identified 23 types of nematocysts in the bodies of Nideria animals. However, four types of nematocysts can be seen in Hydra. Dr. Siddiq Publications
1. Stenotile or penetrant: Stenotile is the largest of the four types of nematocysts of Hydra. Its capsule is filled with a poisonous liquid called hypnotoxin. Its butt is fat. The butt has three large spines. It’s called a barb. Butt has three rows of barbules.
Work of stenotil or penetrant
(i) It paralyzes prey with hypnotoxin.
(ii) Its trigger grips the prey.
2. Streptolin Glutinant or Holotrichus isorhiza: Streptolin Glutinant medium type nematocyst. Its capsule is small and the butt is not well formed. It lacks barbs, but has barbules. Its stems are long, spiny and open at the apex.
Action of Streptolin Glutinant
(i) It traps prey.
(ii) Helps in locomotion.
(iii) secretes sticky substances.
3. Steriolin glutinant or Atrichus isorhiza: Steriolin glutinant is the smallest nematocyst. Their butt is not well formed. Barbs and barbules are absent. Its stems are short, thornless and open at the apex. Dr. Siddiq Publications
Action of stearoline glutenin
(i) It helps in locomotion.
(ii) binds the hydra to an object.
(iii) secretes sticky substances.
(iv) Capture the victim.
4. Volvent or Desmonym: Volvent is a relatively small nematocyst. They do not have butts, barbs and barbules. Its stem is thick, short, elastic, spineless and closed at the apex. There is only one patch of Sutra inside its capsule. The sutraka remains patchy even when thrown. As soon as it is thrown, it creates many screws like a cork-screw.
Volvent’s work
(i) It grasps the prey.
(ii) It helps in locomotion.
Nematocyst initiation technique
Nematocyst initiation is a chemical and mechanical process. When a prey comes close to the hydra’s attractor, the trigger is thrown. Chemicals in the prey’s body increase the water permeability of the hydra’s nematocyst wall. The osmotic pressure or hydrostatic pressure of the capsule increases. Water quickly enters the bag. A chemical called poly-ℽ-glutamate is secreted inside the sac. The operculum opens as soon as the prey touches the nidocile. Sutraka is thrown out with lightning speed. This entire event takes place in just 3 milliseconds.
Once the nematocyst is released, it cannot be returned to the nematocyst. That is, once thrown, it cannot be used again. No more nematocysts are formed in the same nidocyte. Such nidocytes slowly enter the gastrointestinal tract and are mixed with food and digested. Within 48 hours new nidocytes are generated and used.
Coelenteron
The body cavity of animals of the order Nidaria is called a coelenteron. The body cavity of the Hydra is the Cilantron. Archenteron transforms into Cilenteron. It is covered by gastrodermis. It involves extracellular digestion. Food and excreta are transported through it. Hence it is called Gastrovascular cavity. Cilantron is sometimes called blind gut or blind sac. It is exposed through the stoma. Takes food and leaves waste through mouth. Dr. Siddiq Publications
Why cilantro is called digestive circulation
Both alimentary canal and body cavity functions of Nidaria are carried out by cilantrones. Cilenterone performs physiological functions such as digestion, transport of nutrients, respiration, excretion, excretion of waste products, etc. Food material is taken up in the cilantro and extracellular and intracellular digestion is carried out. Undigested food and waste products are excreted through the stomata. So Cilenteron is called Gastrovascular cavity or Gastrovascular cavity. Dr. Siddiq Publications
Importance of cilantro
1. Cilantron contains food.
2. It causes extracellular digestion.
3. Food and excreta are transported through it.
4. It excretes waste through the stoma.
Food intake and digestion of hydra
Nutrition: The biochemical process in which complex food is converted into simple and soluble food and the undigested part is excreted is called nutrition. Nutrients absorbed by the body replenish, grow and produce energy.
Foraging techniques
Hungry hydra grips the base with the soles and floats the body and gills to catch prey. The nematocysts of the attractor are activated when prey approaches. Different types of nematocysts are ejected as the prey touches the attractor.
Volute nematocyst inhibits prey movement. Glutinants trap prey by secreting sticky juices. The stenotil nematocyst injects a poison called hypnotoxin into the victim’s body and paralyzes the victim. Then the food is brought to the mouth. The stomata are swollen and wide and food enters the mouth. Mucus secreted from glandular cells makes food moist and slippery. As a result of contraction and expansion of the hypostome and body wall, food enters the cilantro.
Process of digestion
The biochemical process in which various types of enzymes break down complex food into simple and absorbable food is called digestion. Hydra can digest food such as proteins, fats and simple carbohydrates. They cannot digest starch or complex carbohydrates. Undigested food comes out through the mouth. Hydra digestion takes place in two stages. Extracellular digestion and intracellular digestion.
1. Extracellular digestion: The process in which food is digested outside the cells inside the stomach, alimentary canal and celandine is called extracellular digestion. As soon as the food material reaches the cilentron, the mouth pore closes. The prey or food is killed by the action of the enzyme. The contraction and expansion of the body wall breaks the prey into smaller particles. Food is digested under the influence of enzymes secreted from glandular cells. Proteins are broken down into polypeptides. Lipid food is not digested here. Dr. Siddiq Publications
2. Intracellular digestion: The process in which food is digested inside the cytoplasm of the cell is called intracellular digestion.
Partially digested food particles in the cilantro are converted into smaller particles by the contraction-expansion of the body. Some of the food particles turn into liquid with the help of the passage. The food particles then enter the food cavity in the cytoplasm. Enzymes secreted from the cytoplasm digest food. Food is digested first in an acidic and then in an alkaline phase. Proteins are broken down into amino acids and lipids are broken down into fatty acids and glycerol. Dr. Siddiq Publications
Absorption
Extracellular and intracellular digestion occur in hydra. The digested food is absorbed into the cytoplasm of the transient cells. Absorption of amino acid, glucose, fatty acid, glycerol etc. in the cytoplasm.
Assimilation and exclusion
The digested part of food is called digesta. Nutrients are absorbed in the cytoplasm and transported to different parts of the body by the process of diffusion. Undigested food particles reach the stomata by contraction and expansion of the body wall and flagellar circulation. After that, it is mixed with the stream of water from the mouth. Dr. Siddiq Publications
Different movements of Hydra
The process by which the organism moves under its own efforts due to biological needs is called locomotion.
Movement of hydra: Different types of movement of hydra are looping, somersaulting, gliding, floating, swimming, crawling, stooping, diving and contraction-expansion of the body.
1. Looping
The process by which hydra moves by creating loops is called looping movement. Hydra exhibits looping movements to cover long distances.
(i) In this process the hydra stands upright on the trajectory by the foot.
(ii) Bends the head forward and touches the trajectory by the Karshika.
(iii) A loop is formed between the sole and the head when the base is touched by the nematocyst of Karshika.
(iv) Then pull the feet closer to the head.
(v) Retouches the trajectory by treading.
(vi) Stands straight again with the head raised on the floor.
(vii) Then moves forward bending the head forward.
By forming a loop like this, the hydra moves forward.
2. Somersaulting
Hydra’s fast movement mechanism is somersaulting. Each run creates two loops.
(i) In this process the hydra touches the trajectory by the sole.
(ii) Standing upright by resting on the floor.
(iii) Touches the trajectory by the Karshika with the head facing forward.
(iv) Stand upside down by lifting the feet on the karshika.
(v) Release Karshika by resting on the floor and stand upright with the head up.
Thus the hydra moves forward through digbazi.
3. Gliding or amoeboid movement: Hydra moves by gliding process to cover very short distances. In this process the hydra moves as slowly as the amoeba. A slippery sap is secreted from the cells of the epidermis of the soles of hydra. It creates a moment from the place of the feet. Momentarily moves forward on smooth ground. At the same time, the Hydra moves forward very slowly. In this way, the hydra very slowly crosses a very small distance.
4. Floating: The hydra moves upside down by freeing the feet. That is, the foot is on the top and the oral floor is on the bottom. Gas and mucus are secreted from the glands of the soles of the feet. The released gas and mucus combine to form bubbles. Hydra floats in water with the help of bubbles. During this time, it floats from one place to another due to the pull of the current or the impact of the waves.
5. Swimming: The hydra frees the body and positions itself horizontally. Makes the karshikas move like waves. At the same time, it also moves the body. In this way, it swims by creating a wave-like movement.
6. Walking: Hydra moves upside down by freeing the soles. That is, the foot is on the top and the oral floor is on the bottom. Places the entire weight of the body on the Karshika. Using Karshika as a foot moves slowly. Dr. Siddiq Publications
7. Climbing: Hydra clings to branches of submerged plants by tentacles. Releases and shrinks the soles. Then put the floor in a new place. Thus changes the space through contraction.
8. Contraction and expansion: Hydra relaxes the body and causes contraction and expansion of muscle cells. In this the body size becomes shorter and longer. As a result, a kind of movement is created and the place changes.
9. Drawing: If bubbles are not created in the body, the body becomes heavy. The body sinks faster due to its heaviness. It is called submerged movement. Dr. Siddiq Publications
Respiration
Hydra has no specific respiratory organs. It causes the exchange of gaseous substances through the epidermis and gastrodermis.
1. Epidermis: Dissolved oxygen from the water around the body enters the epidermal cells by diffusion. The absorbed oxygen reaches the various cells of the body and completes the respiration process. Carbon dioxide produced as a result of respiration is released outside the body in the process of diffusion. Dr. Siddiq Publications
2. Gastrodermis: Gastrodermis consists of flagellated cells. Flagellar movement causes a constant flow of water into the stomata. Dissolved oxygen from water enters the gastrodermal cells by the process of diffusion. The absorbed oxygen reaches the various cells of the body and completes the respiration process. Carbon dioxide produced as a result of respiration is released outside the body in the process of diffusion.
Excretion of Hydra
Hydra has no specific excretory organs. Metabolism in cells produces nitrogenous waste products. The waste material generated is released into the water during the diffusion process.
Nerve system of hydra
Hydra is a suborder of animals. Their bodies have a weak nervous system. In the animal world, hydra or nidarians developed the first nervous system. Nervous system emerges from their afferent neurons and joins to form neural network. Neurons do not have axons or dendrites and never form synapses. Mesoglia has one nerve plexus on either side. Nerves are attached to the epidermis and gastrodermis. Nerves are densely located in the mouth and foot. Neurons are connected to each other and to sensory cells and muscle cells. Sensory cells receive light, touch and chemical stimuli from the environment. Then through the nerve network sent to the muscle cells. Dr. Siddiq Publications
Irritability of Hydra
1. Touch: Pricking the hydra with a needle causes the body to contract. Leaves, aquatic plants, objects, etc. are stuck by hydra treads.
2. Hunger: A hungry hydra contracts and expands its muscles. Moves the body at a very fast speed to search for food. If they are not hungry, they slow down.
3. Temperature: Hydra always prefers cool water with a temperature of 20 degrees Celsius. When the surface temperature of the reservoir rises, the hydra slowly moves down. Dr. Siddiq Publications
4. Light: Hydra does not like either too much light or darkness. Always like moderate light. Dr. Siddiq Publications
5. Current: A continuous current causes the hydride body to bend towards the anode. Later, the whole body contracts. Dr. Siddiq Publications
Reproduction of Hydra
The process by which organisms reproduce is called reproduction. Hydra reproduces in two ways. Asexual reproduction and sexual reproduction. Dr. Siddiq Publications
Asexual reproduction: The reproduction that takes place without the union of sperm and egg is called asexual. Asexual reproduction in hydra is – budding and fission.
1. Budding: Budding is an asexual reproductive process in Hydra. This process does not require male or female hydra to reproduce. So it is a simple method. During summer, the environment has more food. Hydra grows physically by consuming food. Then budding occurs in hydra. The higher the number of buds, the faster the number of hydra will grow.
(i) During summer, hydra grows by taking food from the environment and increases in size. Siddique Publications
(ii) Interstitial cells in the middle or lower part of the body divide rapidly to form a small swollen area.
(iii) The swollen part enlarges into a hollow and cylindrical bud.
(iv) Epidermis, mesoglia and gastrodermis gradually develop in bud.
(v) Cilantron gradually expands from mother hydra to bud.
(vi) The bud grows by receiving nutrients from the mother hydra.
(vii) The bud consists of stomata, hypostome and karshika.
(viii) A circular groove is formed at the junction of mother hydra and bud.
(ix) The furrow gradually deepens to separate the offspring hydra from the mother hydra.
(x) Padatal is formed after the disintegration of Aptya Hydra. It then attaches itself to submerged objects and lives independently.
A hydra can produce several buds simultaneously. New buds can be formed from each bud. At this time the matrihydra seems to be a gregarious animal. It takes about three weeks to hatch and live independently from the mother hydra.
2. Fission
When a hydra’s body splits into two or more segments, a new hydra grows from each segment. This is called regeneration. Hydra is divided in two ways. Longitudinal division and transverse division. Dr. Siddiq Publications
(i) Longitudinal division: When the hydra body is divided longitudinally into two or more segments, a new hydra is formed from each segment.
(ii) Transverse division: When the body of hydra is divided transversely into two or more segments, new hydra originates from each segment.
Hydra Sexual reproduction
The process by which sperm and egg unite to form a zygote is called sex. Sexual reproduction in hydra takes place in two stages. Gametogenesis and fertilization.
1. Gametogenesis: The process by which gametes i.e. sperm and egg are produced is called gametogenesis. It is of two types. Spermatogenesis and oogenesis. Dr. Siddiq Publications
(i) Spermatogenesis: The process by which sperm are produced is called spermatogenesis. Spermatozoa are formed on the upper side of the hydra’s body. The interstitial cells of the testis divide repeatedly in the process of mitosis to form spermatogonia. Each spermatogonia enlarges in size by taking food and develops into spermatocytes. Each spermatocyte divides by meiosis to produce 4 spermatids. Each spermatid is transformed into a sperm in the process of spermiogenesis.
(ii) Oogenesis: The process by which eggs are produced is called oogenesis. Ovaries are formed on the underside of the hydra’s body. Ovarian interstitial cells divide repeatedly in the process of mitosis to form oogonia. Each oogonia grows in size by taking food and turns into an oocyte. Each oocyte divides by meiosis to produce 3 small polar bodies and one oocyte. The oocyte transforms into an egg. Ovum is covered with a slippery coat of gelatin.
2. Fertilization: The union of sperm and egg is called fertilization. The spermatozoa break through the nipple of the spermatic cord and come out and swim in the water in swarms. Within 24-48 hours, the sperm unites with the egg to form a zygote.
Development of Hydra
Development: The zygote divides repeatedly and turns into a full-fledged organism is called development. The division of the zygote in Hydra is holoblastic or complete. The stages of its development are as follows.
1. Morula stage: The zygote divides repeatedly by cleavage to become multicellular, sterile and round cells. This is called Marula Dasha. Two types of cells are formed in marula dasha. Micromere and macromere. Dr. Siddiq Publications
2. Blastula stage: In the blastulation process, the cells of the morula stage are arranged in a certain layer and form a hollow spherical structure. This is called blastula dasha. The cavity in the center of the blastula is called the blastocell. The blastular wall is called blastoderm and the cells are called blastomere.
3. Gastrula Stage: In the process of gastulation, the cells of the blastomere form a two-layered nerate and spherical structure. This is called gastrula dasha. The gastrula of hydra is called stereogastula because it is attached to the mother body. The outer layer of the gastrula is called ectoderm and the inner layer is called endoderm. Between the ectoderm and endoderm are jelly-like acellular mesoglia. The gastrular cavity is called the primitive cilenteron.
4. Cyst: Gastula is surrounded by spiny sheath made of chitin. It is called a cyst.
5. Hydrilla: Embryo elongates inside the cyst at favorable temperatures during spring. In the embryo, the mouth opening, hypostome, trachea and foot wheel are formed. This condition of the fetus is called hydrula. Dr. Siddiq Publications
Although hydra is bisexual, self-fertilization does not occur
Most hydra are dioecious, but some hydra are monoecious. Self-indulgence is a sexual process. The union of sperm and ovum of the same animal is called insemination. Even though hydra is bisexual, self-fertilization does not occur. In the same hydra, spermatozoa and ova are produced in two different seasons. In hydra the eggs are immature at the time when the sperm are mature. Again, when the egg is mature the sperm is immature. Therefore sperm and egg of the same hydra can never meet. This is why self-fertilization does not occur in hydra. In hydra, only purification occurs. Dr. Siddiq Publications
Symbiosis
The Greek word Symbioum means live together. The close co-existence of two organisms of different species that benefit from each other is called mytosymbiosis. Algae Zoochlorella and green hydra Chlorohydra viridissima form mitotic organisms. Again, sea urchins and clownfish form a symbiotic relationship. Algae parts are transmitted to the next generation along with hydra eggs. So algae is called lifelong paying guest. How hydra and algae benefit is discussed below.Dr. Siddiq Publications
How Algae Benefit
1. Shelter: Algae take shelter in hydra’s endodermal muscle cells.
2. CO2 Acquisition: Algae use the CO2 produced in hydra respiration for photosynthesis.
3. Nitrogen acquisition: The nitrogenous waste products of hydra metabolism are used by algae to produce carnivorous food.
4. Protection from environmental stress: Algae take shelter in hydra’s body. So it is protected from heat, stress, dryness etc. Dr. Siddiq Publications
How does Hydra benefit?
1. Food: Hydra consumes excess food produced by algae in the process of photosynthesis. When the algae die, the corpse is used by the hydra as food.
2. Obtaining O2: Hydra uses the O2 produced by algae in the process of photosynthesis. Dr. Siddiq Publications
3. Excretion of waste products: Hydra produces nitrogenous waste products in the process of respiration. Algae consume these waste materials and free the hydra waste.
Why is the hydra called a mythical creature?
The close coexistence of two organisms of different species that benefit from each other is called symbionts. Hydra obtains food and oxygen from algae. The CO2 and waste products produced by hydra growth are fed to the algae. As a result, Hydra benefits and life becomes easier. Hence hydra is called a mythic organism. Green hydra called Chlorohydra viridissima forms mitosis.
Hydra Division of Labor
In multicellular organisms, when specific cells, organs or organs perform specific tasks, it is called division of labor. In the animal kingdom, division of labor is first seen in the Nidarians or Hydra. Physiological and cellular division of labor can be seen in Hydra.
Physical division of labor
1. Mouth: Controls intake of food, excretion of waste and flow of water.
2. Traction: Used for locomotion, catching prey, climbing and self-defense.
3. Body: Aids in locomotion and contains buds and genitals.
4. Epidermis: Structures and protects the body.
5. Mesoglia: Forms the base and structure and helps in contraction and expansion of the body.
6. Cilantron: Works on digestion and circulation.
7. Padachakti: Helps to move and keep the body bound.
Cellular division of labor
1. Musculoskeletal Cells: Helps build body armor, locomotion and capture prey.
2. Interstitial cells: Any cell or organ that forms the body.
3. Sensory and nerve cells: Receive and report stimuli.
4. Nutrient cells: carry out extracellular and intracellular digestion.
5. Gland cells: secrete enzymes and sticky substances.
Hydra’s Regeneration ability
Reconstruction of lost or damaged body parts is called regeneration. Hydra has great regenerative capacity due to its totipotency. The Swiss scientist Abraham Tremley (1744) was the first to observe the hydra’s ability to regenerate. Cutting a hydra into pieces creates a hydra from each piece. Karshika and hypostome are formed from the oral end and padachakati from the non-oral end. Dr. Siddiq Publications
1. If the hydra is divided into several segments transversely, each segment creates a complete hydra. Each segment maintains polarity. That is, from the oral end, karshika and hypostome are formed and from the non-oral end, the foot-disc is formed.
2. If the hydra body is divided lengthwise into two parts, a complete hydra is created from each part.
3. If the head of hydra is divided into two parts vertically, one head is created from each part.
The creature is named Hydra after the mythical monster Hydra in Greek mythology. This monster had nine heads. If the strong human Hercules cut off the head of the monster, two heads grew in that place. Dr. Siddiq Publications
Reasons for including Hydra in the phylum Cnidaria
1. In the embryonic stage, the hydra body is divided into ectoderm and endoderm layers. In mature animals, ectoderm transforms into epidermis and endoderm transforms into gastrodermis.
2. In the center of the body is the cilantern or gastrovascular cavity. It is exposed to the outside through the mouth.
3. The body wall consists of interstitial cells. It can be transformed into any other cell. Dr. Siddiq Publications
4. Epidermis consists of nidoblast or nidocyte cells. Nidocyte cells contain nematocysts.
5. Cellular division of labor can be seen.
Hydra is included in the Cnidaria phylum due to the above characteristics.
Why is Hydra called the simplest multicellular animal?
1. Hydra is a bivalve. Its body wall consists of epidermis and gastrodermis. Between the epidermis and the gastrodermis are the acellular mesoglia.
2. Interstitial cells in the body can transform into any other cell.
3. Nervous system emerges from nerve cells to form neural network. It is widespread throughout the body. Dr. Siddique Publications
4. Cilenteron or gastrovascular cavity is present in the body. It acts as a body cavity and digestive tract.
5. Open to the outside through the gastrovascular orifice.
6. Intracellular and extracellular digestion takes place in the body.
7. Absorption and excretion occur in the process of diffusion and absorption.
8. Division of labor is observed for performing physiological and mechanical tasks.
9. Asexual reproduction and sexual reproduction occur in the body.
Hydra is called the simplest multicellular animal due to the above characteristics.